The present invention relates to a transmission apparatus for branching/inserting a Gigabit Ethernet signal, and in particular, to a transmission apparatus for switching transmission paths without instantaneous interruption.
In recent years, broadband lines have spread for Internet services into homes and a demand for lines for IP traffic has been increasing. Accordingly, inexpensive high-speed Gigabit Ethernet has been rapidly spread in a market, in substitution for SONET/SDH, ATM, and so on which were the mainstream of WAN up to now.
The Gigabit Ethernet is a standard compatible with the Fast Ethernet standard, which is widely used in LANs for offices, and layer 2 in the OSI reference model. The types of Gigabit Ethernet signals are roughly classified into two, and there are 1000BASE-X which uses light as a transmission medium and 1000BASE-T which uses Category 5 or higher unshielded twisted pair (UTP) cable as a transmission medium. In particular, the former, 1000BASE-X is widely spread in a market in an access aspect and in a trunk line aspect because it is advantageous in that it can transmit over a long distance and optical devices are low cost.
Here, a Gigabit Ethernet signal is explained in more detail. The Gigabit Ethernet signal is the same as an Ethernet signal of 10BASE-T (a data rate: 10 Mbits/s) or 100BASE-T (a data rate: 100 Mbits/s) in that user data is transmitted in MAC frames defined in sections 2 and 3 of IEEE 802.3z (IEEE 802.3z Type 1000BASE-X MAC Parameters, Physical Layer, Repeater, and Management Parameters for 1000 Mb/s Operation). However, in the Gigabit Ethernet signal in which a data rate is 1 Gbits/s, 8 B10 B code defined in the section 36.2 of IEEE 802.3z is used as code of a physical layer.
In the 8 B10 B code, 256 kinds of data with a code group name being represented by “Dxx.x” and 12 types of special symbols with the code group name represented by “Kxx.x” are defined. A code (order set) with a combination of these codes and special symbols is for use as a flag “/I/” which indicates a null signal state such as an inter-frame gap (IFG) between MAC frames, flags “/S/, /R/, /T/” indicative of the start position of a MAC frame or the end of a frame, or equivalents thereto. In FIG. 2 of JP-A-2007-110457, individual order sets and the meanings thereof are disclosed. These order sets include a code which is called the “Configuration” (/C1/ or /C2/). This configuration is for use in auto-negotiation (AN), which is for exchanging the operation mode or the state of a self device between linked devices. The auto-negotiation is a function that is defined in the section 37 of IEEE 802.3z.
In FIG. 1 of JP-A-2007-110457, a part of an 8 B10 B code table defined by IEEE 802.3z is disclosed. The 8 B10 B encoding refers to a scheme for encoding an 8-bit data into 10-bit data on the basis of a defined conversion table such as shown in FIG. 1 of JP-A-2007-110457. For a single 8-bit data, two types of data are defined, one of which is a 10-bit data stream having more (or equal) logic zeros (“0”s) than (or to) logic ones (“1”s), and the other of which is a 10-bit data stream having more (or equal) “1”s than (or to) “0”s, while eliminating the use of a pattern with the ratio of the numbers of “0”s and “1”s being extremely unbalanced with either of them. This 8 B10 B encoding is a technique for performing encoding while balancing so as to permit the number of 0s to become substantially equal to the number of 1s by the selective use of any one of these two kinds of data patterns, i.e., the data that includes more (or equal) 0s than (or to) 1s and the data including more (or equal) 1s than (or to) 0s, in accordance with an accumulated number difference (the running disparity RD) of 0s or 1s to be contained in a stream of code words which was output until immediately before the encoding. More specifically, the processing is achieved as follows: if 1s are greater in number than 0s in the running disparity (RD) at the time point of interest, then set the RD value to “+,” followed by outward transmission of data on the Current RD+ side in the 8 B10 B code table of FIG. 2 of JP-A-2007-110457; if 0s are more than is in the running disparity, then set RD value to “−,” followed by outward transfer of data on the Current RD− side in the 8 B10 B code table of FIG. 2 of JP-A-2007-110457.
For example, an 8-bit data stream “00001010” is converted into 10-bit data of “010101 1011” when 0s are greater in number in the running disparity at that time (i.e., RD is at “−”) and is converted to 10-bit data of “010101 0100” 1s are greater in number in the running disparity (i.e., RD is at “+”). On a signal receipt side, it is possible to detect the presence or absence of data errors by checking whether this RD value's rule is under compliance.
Here, the RD side of the IFG is described. The code groups constituting the IFG are divided into “/I1/” and “/I2”. “/I1/” is inserted when the RD immediately before becoming the IFG was at “+”, and after encoding of “/I1”, the RD is inverted to be necessarily at “−”. “/I2/” is inserted after encoding of “/1/” when the RD immediately before becoming the IFG was at “−” and after encoding of “/I2/”, the RD necessarily is at “−”. Therefore, even if “/I1/” is inserted on the basis of the immediately preceding RD value, as a result, the IFG becomes the “/I2/” and thus the RD is at “−”.
In FIG. 3 of JP-A-2007-110457, there are disclosed a format of MAC frame defined in IEEE 802.3z and a format of a Gigabit Ethernet signal that is generated by applying 8 B10 B encoding to a MAC frame. MAC refers to the protocol which belongs to layer 2 of the OSI reference model. Exchanging the protocol is defined in IEEE 802.3z. A frame during the exchange of the protocol is called the MAC frame. The role of the MAC frame is to store the protocol and/or data of layer 3 or higher of the OSI reference model in a data field of the MAC frame and also to reliably transfer this stored protocol of layer 3 or higher to a target terminal.
In FIG. 3 of JP-A-2007-110457, the MAC frame essentially includes a preamble (8 bytes) indicative of the beginning of MAC frame, a destination address (6 bytes) indicating a MAC address of a destination terminal of the MAC frame, a source address (6 bytes) indicating a MAC address of a terminal to which the MAC frame is forwarded, a length/type field (2 bytes) indicating the length or type of MAC frame, a data field (variable length) and a check sum value (4 bytes), wherein a data (12 bytes or more) indicative of the null signal state, called the inter-frame gap (IFG) is flown between the MAC frame and a MAC frame adjacent thereto. This data stream of an Ethernet signal with 8 bits being as a unit is converted (8 B/10 B-encoded) into a 10-bit data stream in units of respective bytes, resulting in acquisition of the Gigabit Ethernet signal.
More specifically, a block of 8 bits of each byte is replaced by a 10-bit data stream (codeword, also known as code-group) or by an order set that is an ensemble thereof, while changing IFG to /I/ (idle), the byte at the beginning of MAC frame to /S/, a respective one of the preamble, destination and source addresses, length/type, data, and a check sum value (i.e., frame check sequence or “FCS”) to one of 256 kinds of /D/s (data), and a byte immediately after the MAC frame to either /T/R/ or /T/R/R/. In the Gigabit Ethernet signal, an 8-bit length data segment is converted into 10-bit length data on a per-byte basis, so the rate of a physical layer is 10/8 times greater than 1 Gbits/s, that is, 1.25 Gbps.
In this manner, the Gigabit Ethernet signal is the one that is an 8 B/10 B-encoded version of MAC frame as defined in IEEE 802.3z.
Gigabit Ethernet Add/Drop Multiplexers (ADM) in use for transmission apparatuses using Gigabit Ethernet signals have received attention in views of line efficiency and flexibility of network construction in recent years. The Gigabit Ethernet ADMs perform electrical time division multiplexing on plural Gigabit Ethernet signals so as to improve the efficiency of line use and branch/insert those time division multiplexed Gigabit Ethernet signals at arbitrary nodes. A Gigabit Ethernet ADM branches/inserts each Gigabit Ethernet signal to be contained by switching transmission circuits in a cross connect unit of the device for every Gigabit Ethernet signals.
A configuration of a network using a Gigabit Ethernet ADM will be described with reference to FIG. 1. FIG. 1 is a block diagram of hardware of a network. In FIG. 1, a network 1000 includes a bureau building 100-1, a bureau building 100-3, a bureau building 100-2, and a bureau building 100-4 which are disposed clockwise in a ring shape. The bureau building 100-1 includes a transmission apparatus 200-1 and plural user apparatuses 300 including a user apparatus 300-A. The bureau building 100-3 includes a transmission apparatus 200-3 and plural user apparatuses 300. The bureau building 100-2 includes a Gigabit Ethernet ADM 500 and plural user apparatuses 300 including a user apparatus 300-B and a user apparatus 300-C. The bureau building 100-4 includes a transmission apparatus 200-4 and plural user apparatuses 300 including a user apparatus 300-D.
The Gigabit Ethernet ADM 500 freely changes a transmission path according to a user's convenience to assign a Gigabit Ethernet signal transmitted from the user apparatus 300-A disposed in the bureau building 100-1 to the user apparatus 300-B of the bureau building 100-2 (a solid line+a dashed line) or the user apparatus 300-D of the bureau building 100-4 (a solid line) or to transmit a Gigabit Ethernet signal from the user apparatus 300-C of the bureau building 100-2 together with the Gigabit Ethernet signal from the user apparatus 300-A to the user apparatus 300-D (an alternate long and short dash line+solid line).
As described above, using the Gigabit Ethernet ADM 500 makes it possible to construct a flexible network.
On the other hand, transmission apparatuses are generally required to transparently transmit a transmission signal in view of maintenance and operation. The term “transparent transmission” means that, for example, in the configuration of the network shown in FIG. 1, a state in which the transmission apparatus 200-1, the transmission apparatus 200-3, and the Gigabit Ethernet ADM 500 are physically disposed between the user apparatus 300-A disposed in the bureau building 100-1 and the user apparatus 300-B disposed in the bureau building 100-2 is equal to a state in which the user apparatus 300-A is logically connected to the user apparatus 300-B directly. Specifically, the term “transparent transmission” means that a signal output from the user apparatus 300-A is transmitted to the user apparatus 300-B without changing contents of data or the format of the signal. Even in a transmission apparatus such as a Gigabit Ethernet ADM, it is important to switch transmission paths without generating any transmission errors due to switching, that is, instantaneous interruption.
When a cross connect unit of a Gigabit Ethernet ADM switches communication signals, if switching is performed without monitoring a state of a signal to be transmitted, interruption of a packet during transmission, leakage of data including errors due to non-conservation of running disparity, and transmission to a wrong transmission destination due to combination with other packets may occur. In transmission apparatuses, in particular, apparatuses required to perform transparent transmission, it is required to prevent a wrong transmission of data as described above when transmission paths are switched.
In order to cope with instantaneous interruption occurring when transmission paths are switched, in SONET/SDH, various methods such as a method of calculating an amount of delay due to a path difference between a working system and a protection system and inserting a delay time to the protection system by a memory when switching is performed have been found. However, instantaneous interruption of a Gigabit Ethernet signal according to switching of transmission paths has not been solved until now.
The frame length of a SONET/SDH signal is fixed, while the packet length and IFG length of a Gigabit Ethernet signal are variable. Therefore, it is necessary to monitor a state of transmission data and to perform switching at a timing when the transmission data is not affected.